High pressure reactor polymerization plants convert relatively low cost olefin monomers (generally ethylene, optionally in combination with one or more comonomers such as vinyl acetate) into valuable polyolefin products. Such processes using oxygen or organic free-radical initiators, in particular peroxide initiators, are known in the art and have been used in industry for a long time. The polymerization takes place at relatively high temperatures and pressures and is highly exothermic. The resulting polymer is a low density polyethylene (LDPE), optionally containing comonomers.
High pressure polymerization processes are carried out in autoclave or tubular reactors. In principle, the autoclave and the tubular polymerization processes are very similar, except for the design of the reactor itself. The plants generally use two main compressors arranged in series, each with multiple stages, to compress the monomer feed. A primary compressor provides an initial compression of the monomer feed, and a secondary compressor increases the pressure generated by the primary compressor to the level at which polymerization takes place in the reactor, which is typically about 210 to about 320 MPa for a tubular reactor and about 120 to about 200 MPa for an autoclave reactor.
Modifiers or chain transfer agents are often used in high pressure polymerization processes to reduce the molecular weight and narrow the molecular weight distribution. It is generally known to add modifiers in the suction of the secondary compressor or in the purge of the primary compressor. However, adding modifiers in the secondary compressor can lead to premature thermal polymerization and polymer build-up in the piping of the compressor, which in turn can lead to fouling. Fouling can completely plug gas flow lines in the remainder of the process, which can cause unfavorably high pressure drops, reduced throughput, and poor pumping efficiency in the secondary compressor.
It is also known to add modifier directly to the reactor at a location, but control of modifier concentration within the reactor is challenging and poor control can also lead to fouling. Various methods are known for controlling modifier or chain transfer agent concentration within the reactor. U.S. Pat. No. 6,899,852 discloses a tubular reactor process for obtaining polymers with low haze. The monomer feed streams to the reactor are separated into a transfer agent rich stream and a transfer agent-poor monomer stream, and the transfer agent rich stream is fed upstream of at least one reaction zone receiving the transfer agent-poor monomer stream. The transfer agent-poor monomer stream has 70 wt % or less of the transfer agent relative to the transfer agent rich stream, so as to achieve depletion in the concentration of the chain transfer agent in the downstream reaction zone.
When using a chain transfer agent with a high chain transfer constant in known processes, the residual concentration of transfer agent may be quite low toward the end of the reactor. This can result in production of high molecular weight polymer, causing reduced heat transfer and fouling. Reactor defouls are performed to restore heat transfer so that the process can be operated within the desired temperature window for safety and optimized production rates. Reactor defouls may generally involve removing polymer build-up regularly by mechanical (e.g., hydroblasting or aquadrilling) or chemical (e.g., polymer skin materials) means. Defouls add expense and complexity into the process and create downtime. Other background references include US 2005/192414, WO 2014/046835, WO 2011/128147, WO 2012/084772, WO 2015/100351, and EP 2 636 690 A.
There is a need for processes enabling better control of modifier or chain transfer agent concentration to mitigate fouling and minimize the need for reactor defouls.